To answer the original question. In the inverting configuration an opamp can have a gain less than one or greater than one. In the non-inverting configuration the gain must be greater than or equal to one. You need to pay careful attention when operating from a single supply what inversion means.

If you make a simple 2 resistor voltage divider, set the lower R to 2.5 K ohms (satisfies your min source impedance) and choose the upper resistor to scale the input appropriately.

If what you are measuring has a very high impedance, you might use much higher resistor values and buffer the voltage into your ADC input with a simple follower / buffer to satisfy the minimum signal source impedance, as Bordo suggests.

Sensacell is mostly correct, though the source resistance for a voltage divider is actually with both resistors in parallel (which is the thevenin equivalent). If you can tolerate drawing the 1.6 ma from the 80 volts then an op amp is not required.

If you need draw less then you need to scale the resistors higher and use the op amp. The amp keeps the source resistance the micro sees down to near zero.